The invention relates to corundum abrasive grain having a ceramic coating of from 0.25 to 2 weight percent, based on the weight of the untreated grain, of a ground glass frit, a binder and a fine grain.
Abrasive grain provided with a relatively smooth, adherent ceramic coating formed by a fused-on glass frit and containing, in addition, finely divided metal-oxide particles which are embedded in the coating in such a way that a portion of their surface projects from the coating has been described in U.S. Pat. No. 2,527,044.
European patent application No. 0 014 236 describes a process for the upgrading of aluminum-oxide abrasive grain containing some titanium oxide by the application of a ceramic coating. The coating is fused on and the grain structure is simultaneously altered by transformation of the titanium oxide to the tetravalent oxidation stage during a heat treatment at from 1250.degree. to 1350.degree. C. It is further proposed to add to the coating material silicon carbide (SiC), which at the temperatures used decomposes completely, with formation of CO.sub.2 or CO and SiO.sub.2 or SiO.
In the prior art cited, and also in practice, the increased tenacity with which ceramic-coated abrasive grain is held by resin bonds is attributed to the increase in the specific surface of the abrasive grain and the resultant improved adhesion between the grain and the bond achieved by the use of fine-grained to ultrafine-grained fillers or finely dispersed bubbles when SiC is added.